The present invention relates to radiation detectors and more particularly to proportional counter neutron detectors which are used in proximity to a nuclear reactor. The most widely used proportional counter neutron detector comprises an hermetically sealed member having a high neutron cross-section, neutron absorptive coating on the interior surface thereof, with a centralized electrically conductive collector electrode disposed within the sealed member. The chamber is filled with a mixture of inert gas and selected poly-atomic gases which improve the operational characteristics of the device. An incoming neutron is absorbed in the coating, and atomic particles are emitted which cause ionization of fill gas constituents producing electrons which are drawn to the collector electrode, which is connected to the recording electronic system. A widely used neutron absorptive material is boron-10 isotope, which upon neutron absorption emits charged helium and lithium particles. These particles deposit their kinetic energy in the fill gas upon traversing the fill gas. Ion pairs and electrons are produced from the fill gas, and the electrons are drawn to the positively charged collector electrode wire. With a properly designed counter and applied voltage, the electric field in the vicinity of the center wire is sufficiently high to enable the approaching electrons to make a succession of ionizing collisions, with the fill gas atoms. This avalanching or multiplication leads to the formation of a signal pulse which can be sensed by the external electronic package to register the detection of a neutron capture.
The fill gas typically used is selected to have an adequately low ionization potential, and for its characteristic of producing ion pairs from interaction with the charged particles. The noble gases fit these requirements, and argon is widely used as the noble gas material. It has been found that the signal pulse sharpness, or rate of rise of signal, is distinctly influenced by the electron drift velocity through the gas to the center wire, under the influence of the impressed electric field. The noble fill gases such as argon, offer rather low electron drift velocities. It has been the practice to add small amounts of poly-atomic molecular gases to the main noble gas fill to improve the electron drift velocity characteristic of the fill gas. Typically, the additive gas is present in an amount of about 5 volume percent of the total gas fill. This improvement in drift velocity is traceable to the ability of low-energy electrons to undergo inelastic collisions with the poly-atomic molecules.
The addition of the poly-atomic gas to the fill gas is also desired to serve as a photon absorptive constituent within the ionization chamber. Photons may be produced in the ionization chamber as a result of excitation of the noble gas constituent. Such photons can cause excessive multiplication, which will show up as background noise level in the output electronics, or produce spurious counting events. In the extreme, voltage breakdown may be produced resulting in continuous conduction and inoperativeness of the device. The poly-atomic fill gas should be selected to be highly abosrptive of photon emission, which instead of producing re-emission of photons, results in dissipation of the photon energy by the dissocition of the poly-atomic molecule. It is desirable that the dissociation products of the poly-atomic gas molecule additive have the ability to recombine after dissociation, and commonly used poly-atomic gases which meet this requirement include carbon dioxide and boron trifluoride. It has been discovered that the neutron absorptive active element disposed on the interior wall of the ionization chamber presents a target for competitive combination with the dissociated molecules. This undesired wall combination prevents the desired recombination of the decomposition products back to the recombined molecule of poly-atomic gas additive. Thus, with high accumulated neutron exposure, or even with relatively low neutron exposure, with voltage applied, the recombination property of the gas diminishes. A portion of the dissociated products combine with the neutron absorptive material on the wall rather than with the gas constituent with which it was originally present as a poly-atomic molecule. It is desirable to isolate the boron-10 neutron absorptive material which is deposited on the wall, from the dissociation products of the additive gas. The dissociated gases will be prevented from recombining with the boron-10 coating, and will recombine as the gaseous poly-atomic fill gas molecule. The provision of an isolating coating over the neutron absorptive material to prevent combination of dissociated poly-atomic fill gas constituents with the neutron absorptive coating material can result in significantly improved operating life for the device.